1. Field of the Invention
The present invention relates to new semiconductor films made of metal oxide with large bandgaps formed on substrates.
The new semiconductor films are very useful to be used in electronic and photonic devices.
The present invention relates to also a new method to form said new semiconductor films on substrates made of material which has been used in usual electronic and photonic devices.
2. Related Background Art
Recently so-called power devices such as bipolar transistors, field effect transistors, and thyristors are used in various fields such as domestic electronic articles, cars, machine tools, and illumination. With increase of application, conversion and control of electric power with high efficiency and with a high speed are requested to power devices. Although power devices have been fabricated using silicon (Si) for a long time, limits of silicon devices are predicted. The limits come from the fact that the bandgap of silicon, about 1 electron volt (eV), is small. Research to realize power devices made up of semiconductors with large bandgaps, that is, so-called widegap semiconductors to overcome the limits has been widely done. In particular, development of power devices using gallium nitride (GaN) whose bandgap is about 3.43 eV or silicon carbide (SiC) whose bandgap is about 3.2 eV has been done extensively.
On the other hand, error or trouble of electronic devices due to noise which comes from the cosmic rays or cars and heat has been serious problems. It has been made clear that so-called hostile-environment devices which are proof against a severe environment with noise or heat should be made of semiconductors with large bandgaps. Development of electronic devices using GaN or SiC has been done from these points. However there are many problems to be solved to realize electronic devices made of GaN or SiC.
The most serious problem is that bulk crystal of GaN has not been obtained because an equilibrium vapor pressure of nitorogen is very high relative to that of gallium. Therefore, substrates made up of sapphire or silicon carbide (SiC) are used. GaN can not be formed directly on a sapphire substrate because there is lattice mismatch of 16% between sapphire and GaN. Therefore a buffer layer of aluminum nitride (AlN) is formed on a sapphire substrate before growth of GaN. AlN is resistive because it is difficult to dope impurities into AlN. Use of sapphire substrate in a device which includes multi-layers of semiconductor such as a bipolar transistor and a thyristor is very disadvantageous to their structures and fabrication process. On the other hand, SiC substrate is very expensive because bulk crystal of SiC can be grown at a very high temperature of 2200˜2400° C. GaN devices using SiC substrate or SiC devices are very expensive.
The second serious problem is to realize new devices which can be grown at a lower temperature than that at which GaN or SiC layers are formed. It is necessary to form layers of GaN or SiC at a temperature higher than 1000° C. Large energy is necessary to form semiconductor layers at a high temperature. In addition, there are possibilities that atoms move between layers and a composition is disturbed or dopants move near the interface between layers.
The problems described above can be solved by using molybdenum oxide for such devices. The inventor of the present invention discovered that high quality molybdenum oxide crystal has a large bandgap larger than 3.2 eV and is very useful to be used in photonic and electronic devices (U.S. patent application Ser. No. 10/848,145 and Ser. No. 10/863,288).
However, in the patent application described above, the molybdenum oxide crystal was formed by oxidation of a metallic molybdenum plate. Because the molybdenum plate was not crystal, some fabrication technologies such as cleavage could not used. In addition, it was impossible to integrate the devices formed of molybdenum oxide with those formed of silicon. Furthermore, precise control of a thickness of the molybdenum oxide layer was difficult when it was formed by oxidation of a molybdenum plate.
Therefore it is required to form a layer of semiconductor crystal whose bandgap is larger than 3.2 eV on a substrate made of material which is used in usual devices. The semiconductor layer should be formed at a relatively low temperature such that device structures are not damaged during the formation of the layer. Electronic devices with high withstand voltages and photonic and electronic hostile-environment devices will be made at a relatively low temperature on substrates which are used in devices at present.